1. Field of the Invention
This invention relates generally to voltage translating circuits, and more particularly, to a voltage translating circuit for translating an input signal which is within the range of the supply voltages of the circuit to an output signal which has a greater voltage range.
2. Description of the Prior Art
In general, voltage translating circuits are designed to provide an output signal which is related to, but different from, an input signal which is within the range of the supply voltages of the circuit. In some applications, it is desirable to provide an output voltage which is outside the range of the normal supply voltages of the translating circuit. For example, in a typical monolithic microprocessor having an electrically programmable read only memory (EPROM), the supply voltages of the microprocessor will be +5 volts and 0 volts. However, the on-chip EPROM can only be programmed using a programming voltage well outside of these supply voltages. In a typical N-channel device, the programming voltage will be of the order of +20 volts, while in a CMOS device the programming voltage would be of the order of -15 to +20 volts depending upon the conductivity type of the field effect transistors used to form the EPROM storage cells. If the microprocessor is also of the self-programming type, i.e. capable of automatically transfering the contents of an external memory into the on-chip EPROM, some means must be provided to couple the programming voltage to the EPROM programming logic when the device is forced into the self-programming mode. One convenient way for performing this function is to provide a voltage translating circuit which selectively provides as an output signal the programming voltage when the latter is applied to a particular input pin of the microprocessor. However, prior art voltage translating circuits of this type have tended to be relatively complicated. In addition, such circuits typically require bias voltage generating circuits for biasing the current channel regions of the coupling transistors.